The facial ontogeny of Neanderthals and H. sapiens

While morphological differences between H. sapiens , i.e. Anatomically Modern Humans, and Neanderthals are well known in adults, it is much less clear when and how they arise during ontogeny [1, 2, 3, 4]. While most workers agree that major aspects of their differences are already established in neonates, there is some disagreement about whether differences are further accentuated through divergence and changes in length of postnatal ontogenetic trajectories [5]. Beyond this, how regions of the developing cranium interact during ontogeny in both species and how they differ is limited. To address these, we carry out detailed analyses of postnatal ontogenetic changes in size and shape and of associations between craniofacial regions to test the hypotheses that ontogenetic changes in craniofacial form and interactions between regions do not differ between Neanderthals and Modern Humans. We apply geometric morphometrics to landmark data acquired on 3D surfaces of skulls of H. sapiens and Neanderthals (H. sapiens N=70, Neanderthals N=15) and compare ontogenetic trajectories and associations among developing cranial and facial regions. Multivariate regressions demonstrate that Modern Human and Neanderthal craniofacial growth and development show many similarities, with changes most marked in the anterior maxilla, nasal and zygomatico-maxillary regions. Further, major aspects of the differences between adult Modern Humans and Neanderthals are established early and are therefore already present in the youngest individuals. However, additional differences arise through differences in the degree of change in facial size and significantly divergent trajectories of scaling. In particular, Neanderthals compared to Modern Humans develop a relatively wider face at the level of the zygomatics and a relatively taller maxilla. Considering associations among craniofacial regions in Modern Humans during ontogeny, partial least squares analyses (PLS) show that in infants and juveniles the largest and most significant associations among craniofacial regions are found between the orbit, nasal cavity and maxilla, while in adults, the largest and most significant associations are located more inferiorly, between the maxilla, nasal cavity and palate. Maxillary sinus form shows absence of association with other facial regions except in the infant group where they are closely associated with the overlying maxilla, the orbits and the adjacent nasal cavity. PLS analyses of Neanderthals alone suffer from small sample size but, where significant, suggest that the interactions among cranial components are very similar to those in Modern Humans. PLS analysis of Modern Humans and Neanderthals together do not suggest otherwise. In conclusion, modern humans and Neanderthals share major aspects of craniofacial growth and development, already “in place” at birth, but there are significant differences in the details of how adult form is achieved.